Documentation issues

[ghost]

• Some methods seem to be missing from the docs, such as each widget type's ::new. For example: https://docs.rs/iced/0.3.0/iced/widget/text_input/type.TextInput.html

• Is the example in the readme outdated? The docs suggest there are two ways to use iced, implement either the Application or Sandbox trait, but the example implement neither.

[hecrj]

Duplicate of #340.

[ghost]

What about the readme example?

[hecrj]

The README example is an introduction. The Application and Sandbox traits are introduced later. See #276 for a similar discussion.